This invention relates to a power assisted steering system. More particularly the invention concerns a system of the kind comprising a rack bar having a rack with which a pinion engages so that rotation of the pinion in response to a steering input effects in longitudinal displacement of the rack bar to provide a steering output and in which a piston and cylinder device is located in longitudinal alignment with the rack bar with the piston of the device connected to an end of the rack bar for displacement therewith. With such a system valve means will be provided for determining flow of fluid to the piston and cylinder device and which means is responsive to the steering input to control actuation of the piston and cylinder device and thereby provide power assistance to the displacement of the rack bar which is intended by the steering input.
Steering systems of the type aforementioned are frequently adopted where the rack bar has a so-called "centre take-off" (as disclosed, for example in G.B. Patent Specification Nos. 1,487,315 and 2,071,033A) whereby tie rods from which the steering output is derived are coupled to the rack bar for displacement therewith at a position longitudinally between the rack and the end of the rack bar which is coupled to the piston. With "centre take-off" it is conventional for the rack bar to be extended longitudinally by a piston rod through which it engages on one side the piston of the piston and cylinder device; as a result the piston is out of balance when subjected to equal pressures on its opposite faces because these longitudinally opposed faces have different effective pressurised areas caused by the piston rod area on the one side of the piston. It is therefore conventional to utilise piston rods which are considerably thinner and less robust than the rack bars from which they extend so that the difference between the opposed pressurised areas of the piston is minimised. However, such relatively thin piston rods, whilst adequately withstanding the longitudinal tension or compression to which they are subjected in assisting in the longitudinal displacement of the rack bar, are generally unsuitable for withstanding the side loads or lateral loading to which the rack bar is likely to be subjected in use--particularly from the aforementioned centre take-off. As a consequence it is conventional practice for the rack bar to be slidably displaceable in its hosing and supported by two longitudinally spaced bearings or bushes which are capable of accommodating the side loadings to which that bar will be subjected while a third bearing or bush is provided by which the piston rod os slidably supported and, in effect, a fourth bearing is provided by the sliding engagement of the piston in its cylinder. Furthermore, to accommodate for possible misalignment between the rack bar and the piston rod which may be caused by the aforementioned side loadings from the centre or end take-off or from the driving engagement between the pinion and the rack, it is conventional practice to provide a ball joint coupling between the piston rod and the end of the rack bar from which that rod extends. As a consequence it is recognised that the requirement for the several bearings and the ball joint as discussed above add significantly to the expense of manufacture and size, particularly the longitudinal extent, of the assembly.
It is an object of the present invention to provide a power assisted steering system of the kind discussed above and by which the disadvantages of the prior proposals may be alleviated. The invention therefore aims to provide an assembly of the kind discussed which lends itself to a structure with a relatively shorter longitudinal extent than comparable conventional assemblies, to a structure which has less bearings or bushes than that conventionally regarded as necessary, and to a structure which permits the rack bar to be displaced longitudinally by the piston and cylinder device under control of the valve means at substantially the same speed in both senses of direction so that the power assistance facility which is provided may be substantially the same as that which would be effected from a balanced piston.